The present invention relates to a ball guide and, more particularly, to a ball guide having an exterior component with a cylindrical bore; a shaft freely moving inside the bore in an axial and/or rotational direction; balls for rolling motion between the two components; and a hollow cylindrical cage to retain the balls where the cage is a plastic strip with apertures for the balls, and is coiled around the shaft in the shape of a helix.
U.S. Pat. No. 2,566,421 illustrates a strip material with holes to retain the balls. Furthermore, between each two holes, a cross sectional reduction is provided by notches extending perpendicular to the strip axis. The notches enable the thereby created sections to be bent relative to each other. As an alternative, the patent suggests to produce each section holding one ball, as a separate unit, and to join these individual components by welding. The disadvantage of this is that a bearing constructed of several turns of a coil would not constitute a continuous unitary unit. This may be acceptable in cases where the elements are made of steel or other types of metal as, due to the plastic deformation of the joint areas and the material itself, a minimal amount of coherence may still be obtained. This does not apply however, when the choice of materials is plastics.
Swiss Patent 459,672 illustrates a method for the assembly of a cage made of individual rings, which are mutually cemented or interlocked together. Such plastic cages are manufactured either in single-job production or, for larger batch sizes, as injection-molded parts. The required tooling is rather complex, as an appropriate opening must be provided for each individual ball per ring. Moreover, these openings must be configured so that the balls are secured against inward or outward falling out. Furthermore, it is necessary to produce a separate ring for each cage diameter.
As a whole, the manufacture of the previously mentioned variations is expensive. For each diameter the respective cages must be manufactured or kept in stock.
This invention is based on the objective to propose a ball guide with a cage that, in a simple way, is adaptable to various guide diameters and guide lengths, thereby enabling low-cost production, while still having the rigidity required for a quality guide.
The objective is accomplished by this invention in that the adjacent edges of the helically-wound strip are connected with each other. The advantage of this configuration lies in the fact that cages of various guide diameters and guide lengths may be produced from one single strip, by producing a helix, e.g., by helical coiling to form a cylindrical cage. Additionally, due to the coiling technique, a staggered ball pattern is automatically provided in the longitudinal and transverse directions. This pattern results in high dynamic load capacities because each individual ball will roll along a different line of contact with the shaft and with the external component bore. Also, the pattern provides favorable load distribution. In linear operation, due to the spread of contact points over a multitude of contact tracks, the service life is considerably increased. The connecting technique provides the cage with the necessary rigidity to absorb the guide's operating forces.
Additionally, the strip may have a number of serially arranged ball apertures. This provides the strip with a high degree of adaptability. For larger diameters, however, it is also conceivable for the strip to have two or more rows of balls side-by-side, which can be formed into a helix with its edges connected to each other. Here, the ball retention pockets could, e.g., be mutually offset by one half of their spacing. Such strips should preferably be employed for guides of large diameters.
Preferably, the edges of the strips when wound in a helix are discontinuously connected to each other over their entire length. Joining of the materials is preferred by welding. It is however also conceivable to join the edges by interlocking members. An interlocking connection may be accomplished by grooves and projections running the length of the strip edges, or by flanges running parallel to the strip edges, formed recesses on the other strip edge into which the flange on the strip of the next turn will engage. These interlock configurations enable shifting of the strip edges relative to each other to compensate for diameter adaptation.
To retain the balls in the strip, at least one of the two strip surfaces is provided with projecting lugs on both sides of the strip apertures. Additionally, the balls may be snapped into place. Any additional reshaping of the opening edges, as required, e.g., for brass bushings, becomes unnecessary. This also provides an additional advantage. The risky practice of letting the balls free-up themselves in operation, used for cages whose openings require post-process shaping, does not exist. The effect of this becomes even more pronounced as the ball size decreases, because with smaller balls the risk of jamming is greater than with relatively large balls.
The strip design may be such that, starting with the neutral zone in which the ball centers are held, the strip thickness is dimensioned so that the desired envelopment of the balls is provided by the strip itself. As a result, one strip surface has merely holes from which the balls protrude slightly, while the other surface has lugs projecting from it. Lugs may, however, be provided on both surfaces. Preference is given to the smooth surface, i.e., the one having no lugs, for the exterior surface of the cage facing the bore of the external component.
For improved flexibility of the strip, another configuration provides grooves extending across the width of the strip. The grooves are located between successive ball retention apertures. Preferably, the grooves are provided on both surfaces of the strip. A particularly favorable construction is obtained by selecting polyamide as the plastic material for the strip.
The plastic strip can be continuously produced in a simple way by sectional injection-molding using a form tool. The polyamide material provides a homogeneous transition for the connecting points. This permits simplified tooling. The production costs for a cage are thereby considerably reduced compared with cages produced in one tool, either as a bushing section or a bushing of the finished length with all its openings, by means of injection-molding.
Furthermore, as a supplement to the instruction incorporated in this invention, provisions may be included for one backup ring each, adjacent to both face areas of the cage. At least one back up ring serves as a support base for a compression spring. The compression spring is supported on one side by the backup ring and on the other side by the supporting surface of a shoulder ring located either in the external component or in the shaft.
From the following detailed description taken in conjunction with the accompanying drawings and subjoined claims, other objects and advantages of the present invention will become apparent to those skilled in the art.